Find the sum of all natural numbers between 250 and 1000 which are exactly divisible by 3.
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Solution
Clearly the numbers between 250 and 1000 which are divisible by 3 are 252, 255, 258,________ 999. ∴Tn=999=a+(n−1)d=252+(n−1)3 333=84+n−1 ∴n=333−83=250 ∴S=n2[a+l]=(250/2)[252+999] =125×1251=156375.